Local ventilation and its impacts on urban heat islands and outdoor thermal comfort

Many cities are facing urban overheating issues where the reduction of urban ventilation is one of the key drivers. To address the urban overheating problems, this study concentrates on the analysis of local-scale urban ventilation and its impacts of urban heat islands and outdoor thermal comfort, in order to support wind-sensitive urban planning and design. To achieve this, this study develops a framework for analysing local ventilation, urban heat islands and outdoor thermal comfort with the consideration of local morphological characteristics, external meteorological conditions, local ventilation performance, urban heat islands and outdoor thermal comfort. In particular, the consideration of local morphological characteristics is supported by the development of precinct morphology classification scheme based on three-component protocol of building height, street structure and compactness. Based on the three-component protocol, 20 types of the local ventilation zones were identified in the context of Greater Sydney, Australia.

Field measurement was conducted in three typical local ventilation zones, including open low-rise gridiron, open midrise gridiron and compact high-rise gridiron among the 20, to examine the local ventilation performance, urban heat islands and outdoor thermal comfort in summer 2019. The results indicate that the open midrise gridiron precinct underwent the best precinct ventilation performance, followed by the low-rise gridiron precinct and then the compact high-rise gridiron precinct. The local ventilation created by the sea breeze can help alleviate urban heat islands in the open low-rise gridiron and compact high-rise gridiron precincts with every 0.1 increase in relative wind velocity ratio leading to a 0.09-0.12 °C reduction in UHI intensity. However, in the open midrise gridiron precinct, the local ventilation created by the sea breeze made no difference for urban heat islands. However, the precinct ventilation of the open midrise gridiron precinct still partially exhibited UHI alleviation potential with every 0.1 increase in relative wind velocity ratio leading to a 0.06-0.1 °C reduction in UHI intensity depending on the approaching wind temperature and shading conditions.

Only the precinct ventilation of the open low-rise gridiron precinct leads to outdoor thermal comfort improvement with every 0.1 increase in relative wind velocity ratio leading to 0.29 °C and 0.50 °C physiological equivalent temperature reductions under sea breeze and varying wind conditions, respectively. The results also indicate that within ‘gridiron’ precincts, street orientation is not critical to precinct ventilation performance and its impact on urban heat islands and outdoor thermal comfort. Under wind conditions, trees do not always alleviate urban heat islands and improve outdoor thermal comfort as trees can block sea breeze penetration and inhibit wind cooling potential. These key findings will serve to inform urban heat island mitigation strategies and future planning and design decisions in the built environment.